Mutual conductance tester



Dec. 21, 1948. o. J. MORELOCK I MUTUAL CONDUCTANCE TESTER 2 Sheets-Sheet 1 Filed Oct. 31, 1945 Dec. 21', 1 948.

Filed Oct. 31, 1,945 2 Sheets-Sheet 2 Patented Dec. 2 1 19 48 UNITED STATES PATENT OFFICE MUTUAL CONDUCTANCE TESTER Application October 31, 1945, Serial No. 625,761

13 Claims.

This invention relates to apparatus for measuring the mutual conductance of commercial receiving type vacuum tubes, and particularly to apparatus energized from the customary alternating power and light circuits and of small size for use as a separate unit or for incorporation in a more elaborate tube checker.

Mutual conductance tube testers may be divided into two general classifications, first the type wherein direct current potentials are applied to all electrodes with the exception of the heater, and second those tube testers in which alternating current potential is applied to the anode as well as the heater and which are in general of the portable type. Tube testers in this second classification may have either direct or alternating current grid bias potentials. The potentials mentioned herein are those applied to the various electrodes previous to the application of a signal or a shift in voltage in the grid circuit to obtain a corresponding change in anode current.

Tube testers in the first classification must oi necessity be large and rather bulky to provide space for heavy duty components required in the well regulated direct current supplies for the anode, screen grid, and other electrode potentials. Since the plate resistance of vacuum tubes of the receiving type may vary anywhere from two or three hundred ohms to one hundred thousand ohms, the regulation of the anode and screen supplies is most important. Tube testers of this type therefore are generally limited to permanent or semi-permanent installations and are not practical for portable service.

This invention relates to devices in the second classification. In general there are two methods employed in the manufacture of portable mutual conductance tube checkers. The first of these is the so-called grid shift type in which alternating current potentials are applied to all electrodes, a direct current instrument is connected in the anode circuit, and subsequent differentials of anode current are noted on the direct current instrument when suitable changes in grid potential are made. Various plate voltages and grid potentials can be applied to the electrodes to obtain a measurement on the desired portion of the grid voltage-plate current characteristic curve of the tube. In tube testers of this type to obtain thediflerential of plate current resulting from grid voltage change, it is necessary for the operator to subtract the initial plate current reading from the second orhigher milliammeter reading. This differential plate 2 current is, of course, porportional to the mutual conductance of the tube under test.

An improvement in the alternating current shift type of tube tester comprises a circuit wherein a single reading of the differential plate current is indicated on the direct current instrument. In other words, a reverse current of varying magnitude controlled by a rheostat is caused to flow in the milliammeter circuit whereby the pointer of the milliammeter may be returned to the zero position to cancel out the initial plate current reading. Subsequent shift in grid potential will then give a single indication of proportional mutual conductance, as noted by the magnitude of the meter indication when the grid shift button is depressed.

Another type of alternating current grid shift tube tester comprises a pair of balanced transformer windings. These windings supply anode potentials in series with a dual anode rectifier tube to suppress the alternate half-cycle potentials. A direct current milliameter is connected across a two section balancing potentiometer to provide the completion of a modulation bridge. Theoretically, if the transformer windings are carefully balanced and the potentiometer sections are in exact alignment, the milliammeter will show no initial indication of plate current and will deflect with indications proportional to mutual conductance when an alternating potential of the same frequency as that used to energize the balanced plate windings is applied to the grid electrode of the tube under test. Rectified alternating current potentials must be applied to the screen and grid electrodes previous to the application of the alternating grid potential. There are several disadvantages in this system inthat unless pairs of balanced transformer windings are provided in the device, one plate potential must be used for testing all receiving tube types. In addition, the two windings supplying the plate current energy must be balanced carefully, that is, withinthe order of one percent to eliminate an initial plate current indication. Without careful balancing of these windings, on application of alternating potential to the grid circuit of the tube under test the meter indication will be in error by the amount of initial unbalance reading. A further disadvantage is involved in the use of a two section potentiometer which must track accurately on both sections throughout its full range of rotation. This potentiometer is required for changes in meter sensitivity to cover tests on various types of tubes.

erably a higher, frequency on the tube grid;

More specifically, an object is to provide apparatus for the measurement of the mutual conductance of a tube, the apparatus including a transformer with a plurality of secondary windings for developing energizing potentials for'the tube electrodes, an oscillator also energized from the transformer for supplying a higher frequency signal voltage to the control grid of the tube under test, and a meter connected to the plate circuit of the tube? through a frequency selective network that excludes low frequency currents from the meter.

These and other objects and the advantages of therinvention will be apparent from the following specification when taken with the accompanying drawings in which:

Figure l is a simplified circuit diagram of a mutual conductance measuring apparatus embodying the invention;

Figure 2 is a composite curve showing 'on a time basis the voltages imposed upon the grid and the plate of the. tube under test by an alternating current energizing source;

Figure 3 is a curve showing on a time basis the interrupted pulses of signal voltage;

Figure 4 is a curve showing the instantaneous summation of the bias and signal voltagesapplied: to the'grid; and

Figure dis a curve showing on a'time basis the total plate current including the component resulting from the application of the signal and bias potentials.

The-essential elementsof the novel apparatus for the direct measurernent'of mutual' conduct-" ance include a power transformer T for developing all energizing potentialsa signal source such as that shown within the broken line rectangle A:

ponent due to the applied signal pulse derived,

from the'oscillator A. Commercial embodiments of' the invention will usually includea'plurality' ofsockets for receiving different types of tubes but, for simplicity of illustration and description, the Figure 1 diagram does not show the socket for the illustrated tetrode B nor the sockets, and their circuit connections to transformer T, for receiving other types of tubes.

The power transformerT has a primary wind"- ing l for connection'to' the conventional light and power circuit; and .secondary windings 2, 3, 4 for developing the required heater or filament current, the grid bias voltage, the screen grid stage and the plate voltage respectively forenergi'zing the? tube B; A series inductor 5'isincluded in the plate circuit lead to the tube'B, and the measuringv circuit includes a resistance 6 that is shunted acrossthe plate'circuit inductor 5 through a condenser! havinga low impedance at the signal voltage frequency. The voltage drop across the plate'circuit inductor 5 develops a current through resistor 6 that is measured in a circuit including a 4- resistor 9 and an instrument 8 that may be of any suitable type such as the rectifier meter as shown. A lead Ill connects the secondary windings 2, 3, d to ground the cathode on the heater circuit and to complete the plate-cathode circuit of tube B, the sense of the windings 3, 4 and their connections to the tube elements being such that the gridbiasis' negative when the plate potential is positive.

The grid signal generator A may be of any "desired type and, as illustrated, includes a triode it with a filamentary cathode that is connected across the secondary winding l2 of the transform-er'T', and'gridand plate inductances l3, l4 respectivelycoupled by a magnetic core H5. The plate voltage is taken from the secondary winding 4' through a resistor l6 and switch 11, the plate-cathode circuit being completed by a lead I8 and cathode resistor l9. The grid of the oscillator tube is returned tof the cathode resistor by a-leadflfif; :an d' condensers .21, 22 respectively": are connected across :the grid andplate-"inductances;

Atwinding-Ztpnthe core I5 is connected across; the: resistor 24 to-con's'titute the oscillator .oll li l circuit, and the resistor 24 is c'onnectedbetween:

the gridof tube B andgrid biasingwinding. '3 of the transformerT. A tapsw-itch 25 is preferably associated. with the oscillator. output resistance-24 toypermit adjustment of themagnitudeof'the. signalvoltage applied to the. gridiof the 'tube 'B,

- The method of operation willbe apparentrfroma'consideration-of the several: voltage andcurrent relationships that are-indicated in Figs; '2:

to 5 inclusive; The signal voltagejjgenerator A is operative only; during; thosehal-f cycles of: the. low frequency energizing current during which, the'x plates of 'the signal 'voltage generator tube: l lg'and of the representative tube -B arezat a'positive potential with respect try-their cathodes. and periodic pulses of high frequency. (comparedito the energizing frequency) signal. energy: of the character shown in Fig. 3 are, therefore, developed when the switch l-l is*c10sed-.- The frequency'ofi theasignal voltage is high as 'compared with the frequency: of. "the energizing potentials supplied through transformer T',f;butmay';be and" possibly-51s of the relatively low order of. 5 kilocycles, for a power line frequency-of"50"to' 60 cycles per'second; The essential requirement: is. that. the signaljrequency differ from the power; line energizing: frequency by an amount v:thatcaffords'substantial isolation of power line'and'signal frequencybyrthe frequency selectivebranches? of therplate' circuit of tube; B. An alternating; current bias is applied tothe: grid ofitube B'by transformer winding -3; andfthe voltage on'r the grid therefore "varies cyclically, as 'shownby the: curve of Fig.4. The signal"pulses'are-generated during those alternating half -cycles. when" the grid" bias on tube B' is negative with respect to the, cathode.; and:the plate-potential isp'ositive-.: When switch 1:? is in the open position; causing the signal: voltage generator tobe: inoperative; the grid-plate voltage-relationships of Figi'Z-are' apparent. I

- ,Theplate circuitinductor 5rhas:arelatively*low impedance'atthe frequencyofsthe' energizing source' 'which' is assumed; as indicated inf l igs; 2' to 5; to. be'v60 'cycl'es'zpersecond but the coupling condenser? toftheameasuring circuit'rc rhas ahigh impedance at' that frequency. The? values "of" the inductori andzrcondenser Tare so chosen; intac I c'ordancaw-ith "conventional :de'signt practice; that is energized by closing the switch l1, and pulses of the higher frequency signal voltage are impressed upon the grid of tube B during the halfcycles of negative grid bias. The wave form of the resultant plate circuit current is shown in Fig. 5. This high frequency signal plate current is excluded from the plate inductor and flows through the condenser 'l and the measuring circuit. Thus, the magnitude of the signal current is indicated by the measuring instrument 8. The instrument scalemay .be calibrated directly in values of mutual conductance, as explained hereinafter, when the source voltage is substantially constant or when means, not shown, is provided for adjustin the voltage across the transformer winding I to a constant preselected value.

The voltage regulating means that may be required, if and when the power line voltage is not constant, forms no part of the present invention and therefore no voltage regulating means has been illustrated. It is to be noted, however, that the single adjustment of the input voltage across the primary transformer winding I to a preselected value will condition the apparatus for the accurate measurement of mutual conductance.

If the amplitude of the signal potential is designated as eg, then when this potential is zero, the current in the plate circuit is the normal rectified pulsating current which passes through the low value inductance without any appreciable voltage drop due to the current through this inductance. This current does not flow through resistor 6 because of the relatively low voltage drop across the inductance, and the high impedance of condenser I at the energizing or power line frequency. When, however, the signal potential 6g has some value other than zero, the amplifying property of the tube results in an alternating current component i at the signal frequency, in the total plate current. This component i divides between the inductance 5 and the measuring circuit 69 in accordance with the relative impedances of these parallel paths. Since there is a wide difference in frequency, there is likewise a very substantial difference in impedances of these two paths and the high fre-- quency signal current through inductance 5 is negligible. The current through resistor 6, and likewise the current through the instrument 8 is therefore proportional to the entire alternating current component which results from the signal voltage applied to the grid.

The well-known relation between this current ip and the grid signal voltage e; is:

deflection on the instrument 8 would correspond to 3,000 micromhos; that is Similarly, for a signal voltage of 0.25 volt the full scale deflection of theinstruni-ent will be 12,000-micromhos'. Thus, it is possible to make a multirange device having any desired complement of measuring ranges by either changing the magnitude of the signal voltage or changing the instrument current.

It is to be understood that the invention is not limited to the particular embodiment herein illustrated and described, as various changes that may occur to those familiar with the design and construction of apparatus for measuring tube characteristics fall within the scope of the invention 'as set forth in the claims. For instance, direct current potentials may be applied to the grid, screen and suppressor electrodes of the tube under test, with alternating current applied to the anode through a frequency selective network as shown in section C of Fig. 1. In this case, since direct current potentials would be available for application to the electrodes of the tube under test, the same direct current source might likewise be used for energization of signal voltage generator. Certain advantages would be obtainable with this arrangement, since only moderate currents are involved on these particular electrodes, i. e. screen grid, control grid and suppressor grid, and fair regulation of the direct current source would not be difficult to attain. Higher orders of current involved in the anode circuit of the tube under test would, however, present a more difficult problem and, therefore, the alternating current supply forv the anode circuit is preferred.

I claim:

1. In apparatus for measuring the mutual conductance of a tube having a grid electrode and one or more other electrodes cooperating with a cathode, one of said electrodes being a plate; means for impressing alternating current voltages upon said electrodes to energize the tube, means for imposing an alternating current signal voltage of a different frequency upon said grid, and indicating means responsive only to the plate current component resulting from said applied signal voltage, said indicating means providing a direct indication of the mutual conductance of the tube. V

2. Apparatus for measuring the mutual conductance of a tube having a grid electrode and one or more other electrodes cooperating with a cathode, one of said electrodes being a plate; said apparatus comprising means for impressing energizing voltages on said electrodes, saidmeans imposing a periodically fluctuating potential of relatively low frequency on said plate, means for imposing an alternating current signal voltage of a difi'erent frequency upon said grid, and indicating means responsive only to the plate current component resulting from said applied signal voltage, said indicating means providing a direct indication of the mutual conductance of thetube.

3. Apparatus for measuring the mutual conductance of a tube having a grid electrode and one or more other electrodes cooperating with a cathode, one of said electrodes being a plate; said apparatus comprising means for impressing energizing voltages on said electrodes, said means imposing a periodically fluctuating potential of relatively low frequency on said plate, means for imposing an alternating current signal voltage of a different frequency upon said grid, an anode circuit network having parallel reactive componenets, one component selectively transmitting alternating currents of the plate a eeegsee energizing frequency and :the'lotfi'erwomponent selectively transmitting ialternatingocurrents $01 the-signal vfrequency, -.:and indicating mneansnresponsive to current intheaparal-ldlrcircirit .:that transmits currents/of :theisignalf frequency,

4. In apparatus for mea'suringitheimutualiconduotance of .:a.: tu'beihavingiazigridi ands-1a -IP1E=1364C0- operating with va :cathode, ;mea:ns zforr'impressing alternating lcurrent voltages' upon;saidLgrida-and .plate. to energize 1511811311108, slneans ;f or nimposi'n-g an alternating currentesi'gnal volta'gevofi=acdifierentifrequency uponlsaidigrid, va p1ate-=circuit pedance, and a measuring circuit:coupledaacross at rieasta part of said plate circuit-dmpedanceiby 'a'ireactance' havingaa higltimpedance attheifre *quency of the energizingcpotentialsland .a:.low impedance at the signal ifrequency. ii 5. Inapparatus for measuring themutual ooniductanceof :a-tube :having a gridand a plate cooperating with a :c-athode, energizing aimeanst;

for tsaid tube including:autransrormer havinga primary Winding;nfor zconnection tor a= powerli-ne :and' ia plurality :of secondary windings :for supzp'lying:i-a'lternatingv current? to: heat said :cathode and for. impressing-:alternating :ourrentmoltages ;on:;sa'id -gn-d and plate means .iorimpressingmn alternating -:current: signal voltage: of higher frezquencylon saidagrid a';plateucircuittincluctor of high. impedance :atsthe signal :1 frequency, aaridwa measuring circuit connected acrosssaid plate circuit inductor, csaid measuring :oircuit having-a high rimpedance-zatithe poweriineirequenoy and a relatively low impedanoei-atihe signal 'frequen'cy.

6. In apparatus for measuring the mutualeconductancemf a. tube;;tthe 'inventionsass'reoited in claim 5 :wherein. saidxmeans fortimpressing-z -a*signal voltage onthegrid comprisesca vacuum tube oscillator energized fromisaid transformer;

7. In apparatus .formeasuring.:thezmutualconductanoe of a' tube, the:inventionaaslrecitedin claim 5 wherein said meansforiimpressing SflSlg- :nal voltageon the tube gridxmnprises-a-vacuum tube oscillator having grid andziplate'icircuits, and

,quency when'the platerofnthe tube under-meas- X1139 at rated voltagezand relative' polarity with' 1" C. @voltage :oLa predetermined' relatively low q cy, superimposing upon -thes:contro1 cle- 'sment 'of zthe tube ta Ypredetermined--alternating current voltage of relatively high frequency-=to effect .a- 'change in-the'plate currentof-" the tube under test; and: measuring the change -in plate current which-is h-due to the superimpesed high frequency voltage ztoeindicatema :oharacteristic of :thetube.

-:,-I0;;The; mEthOd ZOfTutGSbingf'ihermifini t-ubes' whichucomprises ienergizingifoheaelements mfthe tube atnratedwoltagezand relative polarityr=with 1A. eC.-;voltage ofta predetermined :relativelyilow 5 frequency;.superimposingrmponiithe:control element .:of .lthe tube a mredetermine'd zaiternating currentwoltage of relatively.% high frequency to effiecteaachangeain the: plate-icurrentraof thei tube underetest; and; measuring the? averageschangerin plate roux-rent Whiehzzis tduertqz the:. superimposed highxifrequeney QC. :voltage on Tithe :control element; l.

.zlL-iAithennnonic tube tester z'comprising eneans i017; energizing the eelements xof; 111118. 3 tube: under test; :at-rtheirrrated moltages with 3A.: 5 0.: voltages of relatively slow 1: frequency, the 1 control element being: energized: at iaepolarityiopposite; toztheiplate zelement,nneans' ior:superimposing' anialtennating :ourrentwoltagetaofea;relatively:higln'frequencyci-on 20 the gridaof T217118 vizube'zunderrtest,-- and meansflfor nneasuringwtheaavenage :chan'ge ineplateeourrent, which isidueatcxthe superimposed high frequency voltageoto dn'dicatesa chara'cteristicrof 'itheFJ-tube. .12;:A nzthermionic :tube atester ecomprising ea transformer =having a irspluizality cof secondaries,

predetermined of said secondaries being-con:

nacted :to tenergizeithe 'aelements 'of a tube -under .test: at theirarated'zvoltages, :the: gXIidLibGi-Hg-COH- nected'zforipolarityopposite-ate the plateran osoil- 51211301?BHEIEgiZBdmfEQIH' another: -s-econdary of said citransformeri :and connected:toisuperimpose' a high irequencyralternating.::current on the grid :of the :tube :llllldelultest, an output :oirou'it adapted *to easeparate'rthe:xplate" current: due to normal energizationraatxrated=rvoltages'eand the plate current l'change'zdue utolthe :highf.frequenoy superimposed -on thesgtid hndzmeans 'fommeasuringthe change cimplateracurrent duto the high frequency superimposed: 011511116 gzzid. v

'7 "133511153 'c-"itester-= ior tthermionic' 'tubes, 'ithe oombinationvofaa transformer wvith'means for adjustfling tithe :appliedtwoltage to the-trans-former pri- ;*mary, vsaid :ftnansiormer having a plurality eof .secondanieszsrone' ofrsaid secondaries being u'tilized ionsnerg'izmg *thesfi-lament' of: the xitube urfder test lzandiarmther for energizing the plate' and grim-the gnidrl'being-trconnected in reverse polar ityito that o ath'ecplate zanaoutput circuitfforth'e plate of Ythe vtube auriderutest having ea pair of branches zwifthsselectiveiimpedancein eaolr-bran'ch for *difzierentiatingxbtween-AII. components of diiferent lirequencypmeans for superimposing on the grid a voltage of different frequeney from that used ffor renergizing rthe'=elements o'fthe tube, means for rectifying oneof zthe selected components 'of the plate current, and means for measuring the rectified component of the plate current. OLIVE-R JAMES MORELOCK.

21179; 048, .Miller NQV.-".7, 113.29 j"2367,868 'JJones ,.-Jan. 23, $19.45 

